where is billboard for mann mission ima stuck

the billboard is inside the wheele chair guy's office

First you talk to luthor bigbucks. get to about 50% with him... after that there will be
new goals.
* find a curator
answer: go to roxie in the party outfit and ask her about it. then you will have to
do.....
* raise $300 for the person roxie found to be the curator...
* pay a museum exhibit
Answer: buy the cheapest piece of art for $500